For operation normal of a cellular telecommunications system, a mobile handset needs to align its frequency with that of a base station by measuring the frequency offset and performing automatic frequency control (AFC) to minimize the frequency drift to the smallest possible range. Thus, in order to compensate for the offset, the frequency offset needs to be first obtained.
Coarse and fine tuning is commonly successively implemented in AFC to correct the frequency offset. Coarse frequency tuning is utilized to narrow a large frequency offset to a relatively small value, while fine tuning is to further correct this to a smallest possible value, thus improving the frequency synchronization accuracy.
Accordingly, two-stage frequency offset compensation by using short sequences for coarse estimation followed by long sequences for fine estimation is commonly employed in AFC. The coarse estimation is low in precision but is effective in estimating frequency offset of relatively large values, whereas fine estimation is efficient in the presence of the offset that is of small value, as it can achieve relatively high frequency tuning precision.
It is known that the frequency offset affects the received signal by the mobile handset by generating phase drift in the sample signal. It is also known that the size of this drift is in proportion to the frequency offset and the duration of the signal sequence. The relationship is defined by the expression Δφ=Δf*T, where Δφ is phase drift, Δf represents frequency offset and T denotes sequence length. Prior art frequency offset estimation schemes typically measure the phase drift in order to derive the frequency offset. It is therefore the phase drift needs to be obtained by the means of comparing the phase of a received signal with that of a reference signal.
Accordingly, there exist several phase drift estimation methods used in TD-SCDMA system, one of which is to use the SYNC-DL (Synchronization Downlink) sequence in the downlink pilot (DwPTS). The content of DwPTS is composed of 64 chips of a SYNC-DL sequence, and 32 chips of a guard period (GP). There are 32 different basic SYNC-DL codes for the whole system.
The SYNC-DL sequence is known to a mobile handset after DwPTS channel synchronization in the cell search upon the mobile handset powering on. Thus, it can be used as a reference signal with which the receiving signal can be compared to derive the phase drift. However, because the SYNC-DL sequence is a short sequence (only of a period of 64-chips), the corresponding drift brought by the frequency offset will not be that distinct. Thus, this scheme is only suitable to make coarse frequency estimation.
A frequency offset fine estimation scheme in the industry is to derive the difference between the phase of the demodulated signal after a joint detection process and that of the signal produced by hard decision device. “Joint Detection” is a key technology in a TD-SCDMA system, which can increase the communication capacity by suppressing the interference from other users. In this method, several signal data (normally 4˜8 symbols) closer to the Midamble will be used for frequency offset estimation. Specifically, data neighboring to the Midamble on the front and back which is more than 144 chips in length is used. Thus, the phase drift caused by the frequency offset may be distinct enough to estimate frequency fine offset. However, as Joint Detection requires numerous computations, this method is not optimal for solely the purpose of frequency offset estimation.